Catalyst for dehydrogenation of alkylaromatic hydrocarbon, process for producing the catalyst, and process for producing vinylaromatic hydrocarbon by using the catalyst

ABSTRACT

A solid catalyst containing a potassium component and an iron oxide component, which is such that the ratio of the pore volume of pores therein having a diameter of from 20 to 100 nm to that of pores therein having a diameter of up to 100 nm falls between 0.7/1 and 0.9/1 and which is stable to oxygen-containing vapor, is favorably used as a dehydrogenation catalyst in producing vinyl-aromatic hydrocarbons from alkyl-aromatic hydrocarbons. The catalyst has high initial activity and good initial selectivity and its life is long. In addition, it is easy to handle. The catalyst may be produced by reducing a potassium component-containing iron oxide composition with hydrogen at a temperature falling between 350 and 600° C., and then oxidizing it with an oxygen molecules-containing vapor at a temperature falling between 250 and 500° C.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a catalyst for dehydrogenation ofalkyl-aromatic hydrocarbons, to a method for producing it, and to amethod of using it for producing vinyl-aromatic hydrocarbons.

2. Description of the Related Art

Heretofore, vinyl-aromatic hydrocarbons such as styrene have beenproduced through dehydrogenation of alkyl-aromatic hydrocarbons in thepresence of a catalyst, for which generally used is an iron oxide-typecatalyst as the catalyst (catalyst for dehydrogenation).

Up to now, various attempts have been made for improving thecapabilities of the catalyst for dehydrogenation, for example, by addingthereto various catalyst components thereby to enhance thedehydrogenation activity of the catalyst and to prolong the lifethereof. In addition, known are methods of pre-treating the catalyst forimproving its capabilities.

Regarding these, we, the present applicant have disclosed a catalyst fordehydrogenation in Japanese Patent Laid-Open No. 178340/1995, which isproduced by reducing an iron oxide-type catalyst that comprises ironoxide and potassium oxide added thereto, with hydrogen at a temperaturefalling between 350 and 600° C.

The catalyst is favorable to production of vinyl-aromatic hydrocarbons,as its initial activity is high and its life is long. However, it hasbecome known that the catalyst generates heat when exposed to air andtherefore must be carefully handled so as to evade its exposure to airwhen filled into reactors.

The present invention is to solve the problems in the related art asabove, and its object is to provide a catalyst for dehydrogenation ofalkyl-aromatic hydrocarbons, which has high activity and goodselectivity for the intended dehydrogenation and has a long life andwhich can be handled with no difficulty, and also to provide a methodfor producing it.

Another object of the invention is to provide a method of using thecatalyst for producing vinyl-aromatic compounds, in which the catalystexhibits high activity and good selectivity while maintaining itscapabilities for a long period of time and can be handled with nodifficulty.

SUMMARY OF THE INVENTION

We, the present inventors have assiduously studied so as to solve theproblems in the related art as above. As a result, we have found that acatalyst to be prepared by reducing a potassium component-containingiron oxide composition with hydrogen at a relatively low temperaturefollowed by oxidizing it with a vapor that contains oxygen molecules hasextremely good capabilities for dehydrogenation of alkyl-aromatichydrocarbons, that the life of the catalyst is extremely long, and thatthe catalyst is stable in oxygen-containing vapors such as air. On thebasis of these findings, we have completed the present invention.

Specifically, the gist of the invention is summarized as follows:

(1) A catalyst for dehydrogenation of alkyl-aromatic hydrocarbons, whichis a solid catalyst containing a potassium component and an iron oxidecomponent and which is characterized in that the ratio of the porevolume of pores therein having a diameter of from 20 to 100 nm to thatof pores therein having a diameter of up to 100 nm falls between 0.7/1and 0.9/1 and that the catalyst is stable to oxygen.

(2) A method for producing a catalyst for dehydrogenation ofalkyl-aromatic hydrocarbons, which comprises reducing a potassiumcomponent-containing iron oxide composition with hydrogen at atemperature falling between 350 and 600° C., followed by oxidizing itwith a vapor that contains oxygen molecules at a temperature fallingbetween 250 and 500° C.

(3) The method for producing a catalyst for dehydrogenation ofalkyl-aromatic hydrocarbons as in above (2), wherein the oxygenmolecules-containing vapor is air.

(4) A method for producing vinyl-aromatic hydrocarbons bydehydrogenating alkyl-aromatic hydrocarbons in the presence of acatalyst, in which is used the catalyst of above (1), or the catalyst asproduced according to the method of above (2) or (3).

DETAILED DESCRIPTION OF THE INVENTION

The invention is described in detail hereinunder.

First described is the iron oxide composition which is the startingmaterial in the method of producing the catalyst of the invention.

The iron oxide composition indispensably contains a potassium component,and generally has the capabilities for catalyzing dehydrogenation ofalkyl-aromatic hydrocarbons. For producing it, for example, employableis a method of mixing iron oxide (Fe₂O₃) and a potassium compound,followed by drying and baking the resulting mixture.

As iron oxide to be used herein, preferred is α-Fe₂O₃; and as thepotassium component, preferred are potassium carbonate and potassiumoxide.

The iron oxide content of the iron oxide composition preferably fallsbetween 40 and 95% by weight, and it is desirable that the compositioncontains from 5 to 30% by weight of a potassium component such aspotassium oxide. The composition may contain potassium ferrite.Potassium ferrite is K₂Fe₂O₄ (or KFeO₂). The presence of potassiumferrite in the composition could be confirmed through X-raydiffractometry.

The iron oxide composition may further contain any other component of,for example, minor alkaline earth metal compounds such as calcium oxide(generally, in an amount of smaller than 3% by weight) and chromiumoxide (generally, in an amount of smaller than 4% by weight), and alsomagnesium oxide in an amount of not larger than about 10% by weight,cerium oxide in an amount of not larger than about 6% by weight, andmolybdenum oxide in an amount of not larger than about 3% by weight.Apart from those, it may also contain rare earth metal compounds such aslanthanum oxide.

For producing the catalyst of the invention, the iron oxide compositionis reduced and then oxidized. The treatment for the catalyst productionis effected in a vapor phase. Prior to the treatment, therefore, it isconvenient to previously shape the iron oxide composition in accordancewith the shape of the intended catalyst. The catalyst may have anydesired shape. In general, it is columnar, having a diameter of from 2mm to 4 mm or so and a length of from 3 to 10 mm or so.

Next described is the reduction with hydrogen of the iron oxidecomposition. The iron oxide composition may be reduced with hydrogen ata temperature falling between 350 and 600° C., but preferably between400 and 500° C. If the temperature for the reduction is lower than 350°C., the reduction rate will be greatly lowered and the intended degreeof reduction could not be obtained. Even if the iron oxide compositionhaving been reduced to such a low degree of reduction is thereafteroxidized in a suitable manner, the resulting catalyst will be readilydeactivated and could not have a long life. On the other hand, if thetemperature for the reduction is higher than 600° C., the activity ofthe resulting catalyst will be low even if the reduced composition isthereafter oxidized in a suitable manner. The time for the reductiondepends on the temperature for it, but is generally not shorter than 1hour, preferably from 5 to 30 hours. When the temperature for thereduction is low, it is recommended to increase the hydrogen flow rateor to prolong the time for the reduction.

Hydrogen to be used for the reduction may contain any other vapor inertto dehydrogenation, such as nitrogen or methane. It may also containsteam, for which the molar ratio of hydrogen to water (H₂/H₂O) ispreferably not lower than 0.05, more preferably not lower than 0.5.

The hydrogen flow rate is preferably not lower than 5 hr⁻¹, morepreferably between 20 and 300 h⁻¹, in terms of the gaseous hourly spacevelocity (GHSV) of pure hydrogen. The pressure for the reduction is notspecifically defined. If desired, the reduction may be effected underelevated pressure or reduced pressure.

Preferably, the reduced composition is such that the ratio of the porevolume of pores therein having a diameter of from 20 to 100 nm to thatof pores therein having a diameter of up to 100 nm falls between 0.7/1and 0.9/1, or that is, the ratio of (pore volume of pores therein havinga diameter of from 20 to 100 nm)/(pore volume of pores therein having adiameter of up to 100 nm)=0.7/1 to 0.9/1. Further in other words, theproportion of the pore volume of pores in the reduced composition havinga diameter of from 20 to 100 nm to that of pores therein having adiameter of up to 100 nm is from 70 to 90%.

Also preferably, the reduced composition has a specific surface area offrom 1.0 to 2.5 m²/g, more preferably from 1.5 to 2.0 m²/g. The specificsurface area and the pore volume as referred to herein may be determinedthrough isovolumetric vapor adsorption.

Next described is the oxidation that follows the previous reduction. Inthe method of producing the catalyst of the invention, the catalystproduced is stabilized to oxygen through the oxidation, whilemaintaining its original capabilities for catalyzation.

The temperature for the oxidation may fall between 250 and 500° C., butpreferably between 300 and 450° C., more preferably between 350 and 400°C. If the temperature for the oxidation is lower than 250° C., thestability to oxygen of the catalyst produced will be poor and thecatalyst will be readily deactivated and could not have a long life. Onthe other hand, if the temperature for the oxidation is higher than 500°C., the activity of the catalyst produced will be low and the catalystwill be readily deactivated. The time for the oxidation depends on thetemperature for it, but is generally not shorter than 1 hour, preferablyfrom 3 to 30 hours. When the temperature for the oxidation is low, it isrecommended to increase the flow rate of the oxygen molecules-containingvapor to be applied to the reduced composition, or to prolong the timefor the oxidation. However, if the temperature for the oxidation islower than 250° C., the effect of the invention could not be attainedeven though the time for the oxidation is prolonged. If the time for theoxidation is shorter than 1 hour, the catalyst produced could not bewell stabilized to oxygen. On the other hand, even if the oxidation iseffected for a longer period of time over 30 hours, no furtherimprovement in the catalyst activity and in the catalyst life could beexpected.

The vapor for the oxidation must contain oxygen molecules. Even thoughcontaining oxygen, a vapor not containing molecular oxygen such as steamis useless in the invention, since the reduced composition as oxidizedwith the vapor of that type could not turn into a catalyst having a longlife. Preferably, the oxygen content of the vapor for the oxidationfalls between 1 and 50% by volume, more preferably between 2 and 22% byvolume. If it is smaller than 1% by volume, the temperature for theoxidation must be high and the time for it must be long. On the otherhand, the vapor having an oxygen content of larger than 50% by volumewill be unfavorable as it is not safe and is difficult to handle.

The vapor for the oxidation may contain any inert gas such as nitrogen,helium and argon. Air is preferred as the vapor, since it is easilyavailable, safe and inexpensive.

The flow rate of the oxygen molecules-containing vapor for the oxidationis preferably not lower than 50 hr⁻¹, more preferably between 200 and800 h⁻¹, in terms of the gaseous hourly space velocity (GHSV) of air.However, if the oxidation vapor flow rate is as above in the initialstage of oxidation, the reaction system will generate heat to have atemperature of higher than 500° C. Therefore, in the initial stage ofoxidation, GHSV of the oxidation vapor must be so controlled that thereaction system is not heated above 500° C. The pressure for theoxidation is not specifically defined. If desired, the oxidation may beeffected under elevated pressure or reduced pressure.

Preferably, the catalyst thus obtained after the oxidation is such thatthe ratio of the pore volume of pores therein having a diameter of from20 to 100 nm to that of pores therein having a diameter of 0 to 100 nmfalls between 0.7/1 and 0.9/1, or that is, the ratio of (pore volume ofpores therein having a diameter of from 20 to 100 nm)/(pore volume ofpores therein having a diameter of up to 100 nm)=0.7/1 to 0.9/1. Furtherin other words, the proportion of the pore volume of pores in thecatalyst having a diameter of from 20 to 100 nm to that of pores thereinhaving a diameter of up to 100 nm is from 70 to 90%.

Also preferably, the catalyst has a specific surface area of from 1.0 to2.5 m²/g, more preferably from 1.5 to 2.0 m²/g. The specific surfacearea and the pore volume may be determined in the same manner as above.

As in the above, it is desirable that the specific surface area of thecatalyst falls between 1.0 and 2.5 m²/g and that the pore distributionin the catalyst is so controlled that the number of micro-pores thereinis small while the number of macro-pores therein is large.

The catalyst for dehydrogenation of alkyl-aromatic hydrocarbons of theinvention is a solid catalyst that contains a potassium component and aniron oxide component. In the catalyst, the ratio of the pore volume ofpores having a diameter of from 20 to 100 nm to that of pores having adiameter of 0 to 100 nm falls between 0.7/1 and 0.9/1. The catalyst isstable to oxygen.

In the catalyst, the potassium component is not specifically defined andmay be any of potassium oxide or potassium carbonate, or may also bepotassium ferrite. The iron oxide component generally comprises ironoxide (Fe₂O₃), and this is the essential component of the catalyst.Regarding their proportions in the catalyst, it is desirable that theiron oxide component is in an amount of from 40 to 95% by weight and thepotassium component is in an amount of from 5 to 30% by weight. If theamount of the potassium component in the catalyst is smaller than 5% byweight, the activity and also the selectivity of the catalyst will bepoor; but if larger than 30% by weight, the activity of the catalyst,especially that in the initial stage of dehydrogenation with it will below.

The catalyst of the invention may further contain any other componentof, for example, minor alkaline earth metal compounds such as calciumoxide (generally, in an amount of smaller than 3% by weight) andchromium oxide (generally, in an amount of smaller than 4% by weight),and also magnesium oxide in an amount of not larger than about 10% byweight, cerium oxide in an amount of not larger than about 6% by weight,and molybdenum oxide in an amount of not larger than about 3% by weight.Apart from those, it may also contain rare earth metal compounds such aslanthanum oxide.

The specific surface area and the pore volume of the catalyst may bedetermined in the same manner as above. As so defined hereinabove, thepore volume ratio of the pores existing in the catalyst must fallbetween 0.7 and 0.9. If the ratio is smaller than 0.7, the catalyst willbe readily deactivated; but if larger than 0.9, the activity of thecatalyst will be unfavorably low. Preferably, the specific surface areaof the catalyst falls between 1.0 and 2.5 m²/g, more preferably between1.5 and 2.0 m²/g. If its specific surface area oversteps the range offrom 1.0 to 2.5 m²/g, the activity of the catalyst will be low.

The catalyst of the invention is stable to oxygen, and this means thatthe catalyst generates little heat when exposed to air. Concretely, thestability to oxygen of the catalyst may be demonstrated as follows: Athermocouple is fixed to the bottom of an evaporating dish having adiameter of 6 cm, the dish with the thermocouple thus fixed thereto iskept in air at room temperature, and 5 g of a sample of the catalyst isput in the dish and left as it is for 10 minutes. Then, the temperatureof the sample in the dish is measured. In that condition, when thetemperature increase after 10 minutes is not larger than 5° C. or so,the stability to oxygen of the catalyst is good.

The catalyst of the invention may be produced, for example, according tothe production method mentioned hereinabove.

In the method for producing vinyl-aromatic hydrocarbons of theinvention, the catalyst for dehydrogenation of the invention mentionedabove, or the catalyst for dehydrogenation as produced according to themethod of the invention also mentioned above is used for dehydrogenatingalkyl-aromatic hydrocarbons to give vinyl-aromatic hydrocarbons.

As the starting alkyl-aromatic hydrocarbons, used are aromatichydrocarbons having 1 or 2 alkyl groups capable of forming vinyl groupsthrough dehydrogenation. Concretely, they include ethylbenzene,diethylbenzene, ethyltoluene, and ethylnaphthalene. In those, thebenzene ring may have any other substituents not participating indehydrogenation, for example, an alkyl group such as a methyl group, anda halogen atom such as a chlorine atom.

Regarding the condition for the dehydrogenation, the temperaturegenerally falls between 550 and 650° C., but preferably between580 and640° C. The pressure generally falls between 200 and 600 Torr, butpreferably between 300 and 500 Torr. The dehydrogenation requires steam,and the ratio of steam to the starting alkyl-aromatic hydrocarbon to bedehydrogenated (steam/alkyl-aromatic hydrocarbon, by weight) generallyfalls between 1.0 and 3.0, but preferably between 1.2 and 2.0. Theliquid hourly space velocity (LHSV) of the alkyl-aromatic hydrocarbongenerally falls between 0.2 and 20 hr⁻¹, but preferably between 0.3 and1.0 hr⁻¹.

As in the above, vinyl-aromatic hydrocarbons are produced fromalkyl-aromatic hydrocarbons, for example, styrene is produced fromethylbenzene.

The invention is described in more detail with reference to thefollowing Examples, which, however, are not intended to restrict thescope of the invention.

In the following Examples and Comparative Examples, used was thefollowing iron oxide composition as the starting material. Also inthose, the following reaction tube was used in preparing catalystsamples and in producing styrene.

First prepared was an iron oxide composition from iron oxide (α-Fe₂O₃)and potassium carbonate (K₂CO₃). Precisely, 450 g of iron oxide and 50 gof potassium carbonate (ratio by weight of the two=9/1) were put into anagate mortar, well mixed therein, and then well kneaded with water addedthereto, and the resulting paste was introduced into a vertical extruderhaving a die size (diameter) of 3 mm and extruded out therethrough. Theresulting molding was dried in air at a temperature falling between 120and 200° C. for 20 hours, and then baked in an air atmosphere in amuffle furnace, at 800° C. for 4 hours to obtain an iron oxidecomposition.

As the reactor tube, a one inch tube (inner diameter: 21.4 mm) having alength of 60 cm was used. 100 cc (132 g) of the iron oxide compositionprepared previously was filled into the tube. The reactor tube wasfilled with {fraction (1/8+L )} inches alumina balls in the upper layersite of the catalyst bed, and the bottom of the catalyst bed was fixedto a perforated plate. The inner temperature of the reactor tube wascontrolled by an electric furnace disposed around the outer surface ofthe reactor tube. The electric furnace had three heating zones separatedfrom each other. A thermocouple protected with a protector tube havingan outer diameter of 6 mm was inserted into the reactor tube, with whichthe temperature in the top, the middle area and the bottom of thecatalyst bed was monitored. The temperature in the top and the bottom ofthe catalyst bed was so controlled that it could be as similar aspossible to the reaction temperature in the reactor tube, by controllingeach heating zone of the electric furnace. In the reactor tube, thereaction was effected in a down-flow system.

EXAMPLE 1

(1) Preparation of catalyst:

100 cc (132 g) of the iron oxide composition prepared previously wasfilled into the reactor tube as above, which was then degassed to have areduced pressure of 460 Torr. Then, nitrogen gas was introduced into thereactor tube at a flow rate of 3 liters/hr. The reactor tube was heatedat a heating rate of 100° C./hr, and when it was heated up to 500° C.,nitrogen gas being introduced thereinto was switched to hydrogen gas.Hydrogen gas was thus introduced into the reactor tube at a flow rate of3 liters/hr (GHSV=30 hr⁻¹), with which the iron oxide composition in thereactor tube was reduced at 500° C. for 24 hours.

After the composition was thus reduced with hydrogen, the temperature inthe reactor tube was lowered to 400° C., and hydrogen gas beingintroduced into the reactor tube was again switched to nitrogen gas.Then, nitrogen gas introduction was effected for 3 hours, and thereactor tube was thus purged with nitrogen. Next, the flowing gas in thereactor tube was switched to air. Air was introduced into the reactortube at a flow rate of 3 liters/hr (GHSV=30 hr⁻¹) for 2 hours, withwhich the composition was pre-oxidized. Then, air was further introducedthereinto at a flow rate of 12 liters/hr (GHSV=120 hr⁻¹), with which thecomposition was oxidized for 10 hours. Through the treatment, thecomposition turned into a catalyst of the invention.

The catalyst thus prepared herein was measured through isovolumetricvapor adsorption, for which was used a measuring device of Belsorp 36(from Nippon Bell). As a result, the catalyst was found to have aspecific surface area of 1.5 m²/g, and a pore volume ratio of poreshaving a size of from 20 to 100 nm of 0.86. The pore volume ratio of0.86 means that the ratio of the pore volume of pores in the catalysthaving a diameter of from 20 to 100 nm is 86% of the pore volume ofpores therein having a diameter of up to 100 nm. 5 g of the catalyst wastaken out in air, and kept in an evaporating dish having a diameter of 6cm, to which was fixed a thermocouple at its bottom, for 10 minutes. Inthat condition, the catalyst having been kept in the dish did notgenerate heat.

(2) Production of styrene:

After the catalyst was produced, air introduction into the reactor tubewas stopped. Then, steam was introduced thereinto at a flow rate of 150g/hr, with heating the reactor tube at a heating rate of 100° C./hr.When this was heated up to 550° C., ethylbenzene was introducedthereinto at a flow rate of 100 g/hr. The ratio of steam/ethylbenzene(by weight) was 1.5, and the liquid hourly space velocity (LHSV) ofethylbenzene was 1.0 hr⁻¹.

Next, the reactor tube was further heated at a heating rate of 100°C./hr up to 620° C., and kept at the elevated temperature of 620° C. for3 months (for which dehydrogenation of ethylbenzene was continued). Theinitial activity of the catalyst, the initial selectivity thereof, andthe deactivation rate thereof were determined. The initial activity ofthe catalyst was indicated by the styrene (SM) concentration (% byweight) in the reaction liquid, and the initial selectivity thereof wasindicated by the ratio of the concentration of styrene formed (SM) tothe concentration of ethylbenzene converted (ΔEB), SM/ΔEB, in terms of %by weight. The deactivation rate of the catalyst was indicated by thereduction in the styrene concentration in the reaction liquid in one day(ΔSM) (wt. %/day). The styrene concentration (SM) and the concentrationof ethylbenzene converted (ΔB) were obtained by analyzing the reactionliquid through gas chromatography. The data obtained are shown in Table1 below.

EXAMPLE 2

A catalyst was produced in the same manner as in Example 1(1). In this,however, the iron oxide composition was reduced with hydrogen in thesame manner as in Example 1(1), and thereafter a mixed gas of air andnitrogen (air/nitrogen=20/80, by volume), but not air only, was appliedto the reduced composition for oxidizing it.

The catalyst obtained herein had a specific surface area of 1.7 m²/g,and a pore volume ratio of pores having a size of from 20 to 100 nm of0.87. The pore volume ratio of 0.87 means that the ratio of the porevolume of pores in the catalyst having a diameter of from 20 to 100 nmis 87% of the pore volume of pores therein having a diameter of up to100 nm. The catalyst was tested for heat generation, if any, in air, inthe same manner as in Example 1(1), but it did not generate heat.

Next, the catalyst was used in producing styrene, also in the samemanner as in Example 1, and its initial activity, initial selectivityand deactivation rate were determined. The data are in Table 1.

EXAMPLE 3

A catalyst was produced in the same manner as in Example 1(1). In this,however, the iron oxide composition was reduced with hydrogen in thesame manner as in Example 1(1), and thereafter this was oxidized withthe inner temperature of the reactor tube being kept at 350° C.

The catalyst obtained herein had a specific surface area of 1.6 m²/g,and a pore volume ratio of pores having a size of from 20 to 100 nm of0.87. The pore volume ratio of 0.87 means that the ratio of the porevolume of pores in the catalyst having a diameter of from 20 to 100 nmis 87% of the pore volume of pores therein having a diameter of up to100 nm. The catalyst was tested for heat generation, if any, in air, inthe same manner as in Example 1(1), but it did not generate heat.

Next, the catalyst was used in producing styrene, also in the samemanner as in Example 1, and its initial activity, initial selectivityand deactivation rate were determined. The data are in Table 1.

Comparative Example 1

A catalyst was produced in the same manner as in Example 1(1). In this,however, the iron oxide composition was reduced with hydrogen in thesame manner as in Example 1(1), and thereafter this was oxidized withthe inner temperature of the reactor tube being kept at 600° C.

The catalyst obtained herein had a specific surface area of 0.8 m²/g,and a pore volume ratio of pores having a size of from 20 to 100 nm of0.92. The pore volume ratio of 0.92 means that the ratio of the porevolume of pores in the catalyst having a diameter of from 20 to 100 nmis 92% of the pore volume of pores therein having a diameter of up to100 nm. The catalyst was tested for heat generation, if any, in air, inthe same manner as in Example 1(1), but it did not generate heat.

Next, the catalyst was used in producing styrene, also in the samemanner as in Example 1, and its initial activity, initial selectivityand deactivation rate were determined. The data are in Table 1.

Comparative Example 2

A catalyst was produced in the same manner as in Example 1(1). In this,however, the iron oxide composition was reduced with hydrogen in thesame manner as in Example 1(1), and thereafter this was oxidized withthe inner temperature of the reactor tube being kept at 150° C.

The catalyst obtained herein had a specific surface area of 3.5 m²/g,and a pore volume ratio of pores having a size of from 20 to 100 nm of0.68. The pore volume ratio of 0.68 means that the ratio of the porevolume of pores in the catalyst having a diameter of from 20 to 100 nmis 68% of the pore volume of pores therein having a diameter of up to100 nm. The catalyst was tested for heat generation, if any, in air, inthe same manner as in Example 1(1), but it did not generate heat.

Next, the catalyst was used in producing styrene, also in the samemanner as in Example 1, and its initial activity, initial selectivityand deactivation rate were determined. The data are in Table 1.

Comparative Example 3

A catalyst was produced in the same manner as in Example 1(1). In this,however, the iron oxide composition was reduced with hydrogen in thesame manner as in Example 1(1), and thereafter steam, but not air, wasapplied to the reduced composition for oxidizing it.

The catalyst obtained herein had a specific surface area of 4.0 m²/g,and a pore volume ratio of pores having a size of from 20 to 100 nm of0.65. The pore volume ratio of 0.65 means that the ratio of the porevolume of pores in the catalyst having a diameter of from 20 to 100 nmis 65% of the pore volume of pores therein having a diameter of up to100 nm. The catalyst was tested for heat generation, if any, in air, inthe same manner as in Example 1(1), but it did not generate heat.

Next, the catalyst was used in producing styrene, also in the samemanner as in Example 1, and its initial activity, initial selectivityand deactivation rate were determined. The data are in Table 1.

Comparative Example 4

Styrene was produced in the same manner as in Example 1. In this,however, the iron oxide composition that had been used as the startingmaterial in the Examples and the Comparative Examples mentioned abovewas used as the catalyst. The initial activity, the initial selectivityand the deactivation rate of the catalyst used herein were determined.The data are in Table 1.

The iron oxide composition used herein as the catalyst had a specificsurface area of 2.5 m²/g, and a pore volume ratio of pores having a sizeof from 20 to 100 nm of 0.69. The pore volume ratio of 0.69 means thatthe ratio of the pore volume of pores in the catalyst having a diameterof from 20 to 100 nm is 69% of the pore volume of pores therein having adiameter of up to 100 nm. This was tested for heat generation, if any,in air, in the same manner as in Example 1(1), but did not generateheat.

Comparative Example 5

The iron oxide composition was reduced with hydrogen in the same manneras in Example 1. Without being oxidized, this was used in producingstyrene.

The catalyst obtained herein had a specific surface area of 1.6 m²/g,and a pore volume ratio of pores having a size of from 20 to 100 nm of0.86. The pore volume ratio of 0.86 means that the ratio of the porevolume of pores in the catalyst having a diameter of from 20 to 100 nmis 86% of the pore volume of pores therein having a diameter of up to100 nm. The catalyst was tested for heat generation, if any, in air, inthe same manner as in Example 1(1). In this test, the temperatureindicated by the thermocouple varied from 20° C. to 80° C. This meansthat the catalyst tested generated much heat.

Next, the catalyst was used in producing styrene, also in the samemanner as in Example 1, and its initial activity, initial selectivityand deactivation rate were determined. The data are in Table 1.

TABLE 1 Properties of Catalyst Heat Condition for Oxidation InitialInitial Generation in Oxidizing Temperature Activity SelectivityDeactivation Exposure to Agent (° C.) Time (hr) (wt. %) (wt. %) Rate(%/day) Air Example 1 air 400 10 71.1 96.1 −0.034 no Example 2air/nitrogen 400 10 71.2 95.9 −0.032 no Example 3 air 350 24 71.0 96.0−0.033 no Comparative air 600 10 67.9 96.0 −0.051 no Example 1Comparative air 150 24 70.3 95.8 −0.063 no Example 2 Comparative steam400 10 69.8 95.3 −0.068 no Example 3 Comparative — — — 69.6 95.4 −0.083no Example 4 Comparative — — — 71.2 96.1 −0.033 yes Example 5

From Table 1, the following were confirmed:

(i) The catalysts as produced from an iron oxide composition throughreduction with hydrogen followed by oxidation at 400° C. (Examples 1 and2) or at 350° C. (Example 3) are much better than the catalyst of theiron oxide composition itself not subjected to reduction and oxidation(Comparative Example 4) in that the initial activity of the former ishigher in some degree, the deactivation rate of the former is muchsmaller, or that is the deactivation rate of the former is from{fraction (1/3+L )} to {fraction (1/2+L )} of that of the latter, andthe selectivity of the former is higher.

(ii) When an iron oxide composition is reduced with hydrogen and thenoxidized at a high temperature (600° C.), as in Comparative Example 1,the resulting catalyst is not good since its deactivation rate is highand its initial activity is low.

(iii) When an iron oxide composition is reduced with hydrogen and thenoxidized at a low temperature (150° C.), as in Comparative Example 2, agood catalyst could not be obtained even though the time for oxidationis 24 hours. The deactivation rate of the catalyst obtained is large.

(iv) When steam is used as the oxidizing agent, a good catalyst couldnot be obtained. The initial activity of the catalyst obtained islowered in some degree, and the deactivation rate thereof is large(Comparative Example 3).

(v) The catalysts of such that the ratio of the pore volume of porestherein having a diameter of from 20 to 100 nm to that of pores thereinhaving a diameter of 0 to 100 nm falls between 0.7/1 and 0.9/1 (Examples1 to 3, Comparative Example 5) have high initial activity and goodinitial selectivity, and their deactivation rate is small. However, thecatalyst obtained in Comparative Example 5 generates much heat whenexposed to air, and must be handled with care.

(vi) The catalysts having a specific surface area of from 1.0 to 2.0m²/g (Examples 1 to 3, Comparative Example 5) have high initial activityand good initial selectivity, and their deactivation rate is small.However, the catalyst obtained in Comparative Example 5 generates muchheat when exposed to air, and must be handled with care.

As demonstrated hereinabove, the catalyst of the invention and also thecatalyst as produced according to the production method of the inventionhave increased initial activity and increased initial selectivity andtheir life is much prolonged, as compared with any other catalystsoverstepping the scope of the invention. In addition, the stability tooxygen of the catalysts falling within the scope of the invention ismuch increased. Therefore, the catalyst of the invention and also thecatalyst as produced according to the production method of the inventionare extremely favorable to dehydrogenation of alkyl-aromatichydrocarbons.

In the method of producing vinyl-aromatic hydrocarbons of the invention,the catalyst falling within the scope of the invention is used fordehydrogenating alkyl-aromatic hydrocarbons. Therefore, according to themethod, vinyl-aromatic hydrocarbons are efficiently produced for a longperiod of time. In addition, the catalyst to be used in the method ishighly stable to oxygen, and is easy to handle.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. In a catalyst for dehydrogenation of alkali-aromatic hydrocarbons comprising a solid catalyst containing a potassium component and an iron oxide component and which is characterized in that the ratio of the pore volume of pores therein having a diameter of from 20 to 100 nm to that of pores therein having a diameter of from 0 to 100 nm falls between 0.7/1 and 0.9/1, the improvement wherein said catalyst has been pretreated by reducing a potassium component-containing iron oxide composition with hydrogen at a temperature of 350 and 600° C., followed by oxidizing it with oxygen containing vapor at a temperature of 250 and 500° C., the catalyst being stable to oxygen.
 2. A method for producing vinyl-aromatic hydrocarbons, comprising dehydrogenating alkyl-aromatic hydrocarbons in the presence of the catalyst of claim
 1. 3. A method for producing a catalyst for dehydrogenation of alkyl-aromatic hydrocarbons, which comprises reducing a potassium component-containing iron oxide composition with hydrogen at a temperature falling between 350 and 600° C., followed by oxidizing it with a vapor that contains oxygen molecules at a temperature falling between 250 and 500° C.
 4. The method for producing a catalyst for dehydrogenation of alkyl-aromatic hydrocarbons as claimed in claim 3, wherein the oxygen molecules-containing vapor is air.
 5. A method for producing vinyl-aromatic hydrocarbons, comprising dehydrogenating alkyl-aromatic hydrocarbons in the presence of the catalyst prepared by the method of claim
 4. 6. A method for producing vinyl-aromatic hydrocarbons, comprising dehydrogenating alkyl-aromatic hydrocarbons in the presence of the catalyst prepared by the method of claim
 3. 